Basic Probability help

It is known that the probability of selecting an adult over 40 years of age with cancer is 0.05. The probability of a doctor correctly diagnosing a person with cancer as having the disease is 0.78 and the probability of incorrectly diagnosing a person without cancer as having the disease is 0.06.

What is the probability that a person diagnosed as having cancer actually has the disease?

I'm a bit of a loss as far as setting up this problem goes. The probability of a person diagnosed as having cancer is Pr(A) = 0.78+0.06 = 0.84, right? And we know that the probability of a person having cancer is Pr(B) = 0.05.

So, I would guess that I'm looking for Pr(A|B) = (0.84*0.05)/0.05 = 0.84

I think you need to use the law of total probability.
Let be the probability that the person has cancer, so is the probability that the person doesn't have cancer.
And the probability that the person gets diagnosed, then;

hmm, actually, i dont know where i was going with this :P let me think about it a little more...